Semiconductor devices using metal-semiconductor barriers, known as Schottky barriers, instead of p/n junctions, have been developed to convert incident light into electrical energy. Silicon is often used as the semiconductor in Schottky barrier photodetectors for detecting in the infrared portion of the electromagnetic spectrum. Schottky barrier infrared photodetectors are well known in the art.
Examples of prior art Schottky barrier infrared photodetectors are disclosed in U.S. Pat. No. 4,531,055 to Shepperd Jr. et al and in U.S. Pat. No. 4,533,933 to Pellegrini et al. These prior art examples are planar devices in which the Schottky barrier photodetectors are comprised of a thin metal film, which could be a metal silicide film, on silicon. Normally incident light passes through the Schottky barrier once, which only absorbs a portion of the light leading to low external quantum efficiency levels.
To increase the absorption of the incident light, and hence, the external quantum efficiency, several solutions have been put forth. In U.S. Pat. No. 4,876,586, Dyck et al disclosed increasing the number of passes through the metal-semiconductor interface, and hence, the optical absorption, by selectively etching a 100 silicon wafer along the 111 planes to create a corrugated surface on which the metal was deposited. The corrugated surface, which is created in a similar manner as silicon V-grooves, increases the number of passes through the Schottky barrier through lateral diffraction of the incident light, In U.S. Pat. No. 4,394,571, Jurisson disclosed placing a mirror at a quarter wavelength from the metal-semiconductor interface to enhance the optical absorption of the device. He claimed that, at the design wavelength, most (about 95%) of the light will be absorbed. However, the absorption will not be uniform over a wavelength range, as has been noted by Elabd and Kosonocky in “Theory and Measurements of Photoresponse for Thin Film PdSi and PtSi Infrared Schottky-Barrier Detectors With Optical Cavity,” RCA Review 43, pp. 569–589, 1982 and by Mercer and Helms in “A diffusion model for the internal photoresponse of PtSi/p-Si Schottky barrier diodes,” J. Appl. Phys. 65 (12), 15 Jun. 1989. In each case, the authors reported that they investigated Schottky barrier photodetectors with an optical cavity created by a mirror over a range of wavelengths and found them to be wavelength dependent.
It is also possible to increase the optical absorption in a Schottky barrier photodetector by inducing a surface plasmon mode at the metal-semiconductor interface. This was achieved by refracting the incident light by a semicylindrical lens, according to U.S. Pat. No. 5,685,919 to Saito et al. In this case, increased absorption is only achieved at the correct angle of incidence of the light. According to U.S. Pat. No. 5,625,729 to Brown, coupling of the incident light to the surface plasmon mode was also achieved by use of a grating, which also requires the incident light to be at the correct angle. The angle of incidence required for both structures varies with the wavelength of the incident light, therefore at specific angles these devices will be narrow band detectors. To be broadband, they need to incorporate some kind of method of adjusting the incident angle of the light depending on its wavelength.
All of the above mentioned prior art require that the incident light be either normal or at a specific angle to the metal-semiconductor interface, making integration with optical fibers and other optical waveguides in an integrated optical circuit difficult. Yang et al in U.S. Pat. No. 4,857,973, propose a solution to this integration problem, They integrate a silicon channel waveguide with one or two Schottky barrier photodetectors. The Schottky devices are placed above and below the waveguide so that the “tail” region of the optical mode interacts with them and is absorbed. They claim that up to 70% of the incident light can be absorbed by these photodetectors. This is still not entirely satisfactory since at least 30% of the incident light is lost.
The contents of the above-mentioned technical articles and US patents are incorporated herein by reference, and the reader is directed to them for reference.